''joSIiLi. oS! of the Ovum of the Pike. 203 
commenced to beat : the flow is therefore independent of the heart's 
action at this period. 
The embryo is nourished by the vitellus contained in its yelk- 
bag until the time when the mouth is open and prey can be 
caught. 
The heart was observed to beat before blood-corpuscles were 
within its cavity, showing that its contractions at that time at 
least did not depend on the stimulus afforded by the blood-cor- 
puscles. 
The respiratory changes in the blood were effected, in the first 
place, in the veins on the surface of the yelk-bag, and respiration 
took place at first before the blood reached the heart. This is the 
permanent condition amongst the Mollusca. After a time, the 
partial absorption of the yelk rendering the exposing surface less, 
and the increased size of the fish, demand more extensive methods 
of aerating the blood. This is attained by the formation of the 
gills ; and then the respiratory changes occur after the blood has 
passed through the heart. 
In Mammals, after birth, and in birds and reptiles after extru- 
sion from the egg, a complete change in the course of circulation 
and in the manner of respiration necessarily takes place. This does 
not occur in the case of the fish : the circulation and respiration are 
the same after as before extrusion from the egg ; that is, so far as 
any immediate change is concerned. 
The blood-vessels are laid out independently of the heart ; they 
do not grow out from it like the branches of a tree, but are formed 
in their several localities, and are afterwards united to each other, 
and thus to the centre of circulation. 
A remarkable change takes place in the system of blood-vessels, 
whereby the direction of the current is reversed in a main vessel by 
the obliteration of its origin — a somewhat parallel but not analogous 
case to the obliteration of the ductus arteriosus in the Mammal, and 
the diversion of the blood-current into another channel. 
In other respects the adult condition of the circulation appears 
to be the same as that gradually forming in the course of develop- 
ment of the embryo. 
